I'm suprised no one has mentioned:
The Ark of the Covenant
R2-D2
I'd also still have to go with a lightsaber. I've had multiple experiences with people who aren't prop geeks who have instantly recognized my lightsaber hilts, including a door-to-door salesman, the owner of a company I worked for, neighbors, and my kids' friends. Sure it might look like a piece of pipe, but you throw grips and a d-ring on it, and people know what it is.
After that, I'd have to go with Vader's bucket, a stormtrooper helmet, the flux capacitor, and the headpiece to the staff of Ra.